1. Field of the Invention
This invention relates to an image processing method by which an output corresponding to color document information can be obtained and an apparatus which forms a color image on an image retainer electrophotographically, such as a color reproducing machine, a color printer, and the like.
2. Description of the Prior Art
Various methods have been known in the past to read a color document with a scanner and to perform color extraction of the color document thus read.
(1) First method:
As depicted in FIG. 8 of the accompanying drawings, a color document 101 is irradiated with red and blue lights emitted from red and blue light sources 102 and 103, respectively, and the respective optical information is received by photoelectric conversion means 104 such as CCD to convert them into electric signals. Then, the outputs of the photoelectric conversion means 104 are normalized to V.sub.R and V.sub.B, respectively, by a white paper output value, and these two signals are processed to determine a color extraction map. Preprint No. 1107 of the 1982 general Meeting of the Institute of Electronics and Communication Engineers of Japan suggests that a color extraction map as shown in FIG. 9 of the accompanying drawings can be prepared and a plurality of colors can be extracted on the basis of this map. In FIG. 9, the ordinate represents the normalized output (%) of the photoelectric conversion means 104 when the red light source is turned on and the abscissa does the normalized output (%) when the blue light source is turned on.
(2) Second method:
Two photo-detector parts having mutually different spectral sensitivities are disposed for one pixel, and the outputs V.sub.A and V.sub.B of these parts are processed to extract colors (refer to Japanese Patent Laid-Open No. 44825/1982). For instance, judgement is made in the following manner for a luminance axis in a longitudinal direction (V.sub.A +V.sub.B) EQU white when V.sub.A +V.sub.B .gtoreq.a.sub.1, EQU chromatic colors when a.sub.2 &lt;V.sub.A +V.sub.B &lt;a.sub.1, and EQU black when V.sub.A +V.sub.B .ltoreq.a.sub.2.
Similarly, judgement is made in the following manner for a hue axis in a transverse direction (log V.sub.A -V.sub.B): EQU red when log V.sub.A -log V.sub.B .gtoreq.b.sub.1, EQU green when b.sub.2 &lt;log V.sub.A -log V.sub.B &lt;b.sub.1, and EQU blue when log V.sub.A -log V.sub.B .ltoreq.b.sub.2
(where each of a.sub.1, a.sub.2, b.sub.1 and b.sub.2 is a constant).
FIG. 10 shows the color extraction map obtained in this manner.
(3) Third method:
This is a method which separates color information into red, green and blue by use of a dichroic mirror and prisms or R, G and B filters (see Japanese Patent Laid-Open No. 62320/1975).
FIG. 11 shows various color separation methods. FIG. 11(a) shows a construction wherein an image 112 of a camera lens 111 is separated into three colors by use of a plurality of relay lenses 113 to 116 and dichroic mirrors 117,117' and the separated images are again formed on camera tubes 118 to 120. FIG. 11(b) shows a construction wherein a plurality of prisms 121 to 124 having specific shapes are disposed between the camera lens 111 and the camera tubes 118 to 120, respectively, while dichroic mirrors 117, 117' are disposed between the prism 121 and the prism 122 and between the prism 123 and the prism 124 respectively, for the color separation into the three colors.
FIG. 11(c) shows a construction wherein three prisms 125,126,126', each having an apex of an acute angle, are fitted in such a manner as to describe a triangle ABC as shown in the drawing, and dichroic mirrors 127,128 are formed on the boundary surface of each prism for the three color separation. FIG. 11(d) shows a construction obtained by exactly turning inside out the construction of FIG. 11(c). Dichroic mirrors 129, 130 are formed on the boundary of each prism.
The first method uses the normalized output of the photoelectric conversion means 104 as such for the axis of the coordinates of the color extraction map coordinates system as shown in FIG. 9. Accordingly, there is no distinction at all between the chromatic colors and achromatic colors and the concept is unclear. Furthermore, there is no concept, either, of the luminance and hue of the document. Since there is no concept of converting the reflection density or reflection rate of the document into the output level, it is difficult to establish correspondence between the document and the output level.
Incidentally, color systems includes a Munsell color system. This system represents three-dimensionally a given color in the coordinates of lightness (value), saturation and hue as shown in FIG. 12. Since the example shown in FIG. 8 does not have the concept of the lightness (value), a given color is viewed on a cross-sectional plane of certain lightness, or in other words, the full lightness is projected on a hue-saturation plane.
The second method does not at all consider a monochromic document. In other words, since this method does not at all have the concept of an achromatic color (grey level), the method is extremely unrealistic when a practical output result is taken into consideration. As to the color document, too, this method is not easy to practise because it does not have the concept of the correspondence between the document and the output value. In other words, this method does not have the concept of converting the reflection density or reflection ratio into the output value level.
The third method has excellent color extraction ability because it is used in the printing field, too, but involves the problem that the construction is complicated as shown in FIG. 11 Therefore, a large number of limitations exist when color separation means having such a construction is incorporated in a reproducing machine, or the like. For instance, since a scanning mirror reciprocates on the color separation means, a large unnecessary space is required so that the size of the apparatus is increased and a regulation mechanism is complicated, too, with an enhanced production cost.
Since color display devices such as color CRT's for computers, videotex terminals, and the like, have become widespread in recent years, the demand for a color hard copy is also increasing. Various business documents are now color-printed, either fully or partially, and the demand for color copies is increasing, too.
Preferably, means for obtaining color hard copies is economical, has a high operation speed and is easy to handle.
Wire dot matrix recording system, ink jet recording system, thermo-sensitive transfer recording system, electrostatic recording system, electrophotographic system, and the like, have been developed to satisfy these requirements.
When various recording systems described above are compared with the electrophotographic system, their merits and demerits are as follows.
(1) Wire dot matrix recording system:
This is a method which prints a character or image consisting of a matrix dots obtained by hitting the back of an ink ribbon by dot wires. This method can reduce the size of the recording apparatus and can be produced economically, but is not free from the problems that the printing speed is low and large noise is generated.
(2) Ink jet recording system:
This system constitutes a character or image by dots obtained by ejecting very small ink droplets Though the recording apparatus for this system can be made relatively compact, it is difficult to produce and necessitates a multi-element nozzle head consisting of a plurality of very expensive elements in the linear form in order to increase its operation speed. Another critical problem of this system is that clogging of the nozzle is unavoidable and hence reliability of the recording apparatus cannot be kept for an extended period. Though a valve jet system has been developed to solve these problems, it is not yet completely satisfactory.
(3) Thermo-sensitive transfer system:
In accordance with this system, a thermal head formed by arranging minute heat generating elements is brought into contact with the rear of an ink film which is fused and transferred to paper upon heating, and electric power is applied to the heat generating elements in order to fuse or sublimate dyes on the ink film and transfer them to a recording medium for recording. Since this method does not use a developing device which deals with powder inside the apparatus, the system is clean and has a simple construction. However, it is not free from various problems. Namely, since the ink film is affected by the quality and condition of the paper to which transfer is made, there is a limit to the kind of paper. Next, the greater the number of colors to be recorded, the greater the number of the ink films, so that running cost will become very high when the ink films cannot be reused. Furthermore, the resulting image looks unnatural because luster is stressed abnormally, and a thermal head capable of high speed operation is very expensive.
In accordance with the three systems described above, it is extremely difficult to obtain resolution of at least 16 dots/mm because there is a limit to the thickness of the dot wires in the case of the wire head matrix system (1) and there is a limit to the sizes of the nozzle and the heat generating elements of the head in accordance with the systems (2) and (3).
In contrast, the apparatus in accordance with the electrophotographic system cannot be made compact so easily because it includes a developing device which deals with a photosensitive member and powder or a liquid, but it has an advantage that high resolution of at least 16 dots/mm can be obtained easily and recording can be made at a high operation speed.
Electrophotographic reproducing machines such as "Xerox 6500" of Xerox, "NP-COLOR, T" of Cannon and "RICOH COLOR 5000" of Ricoh, which are referred to as a "full color processor" and reproduce the colors of the document as such, have the following construction. Namely, the photosensitive member is passed through a green (G) filter, for example, after corona charge to expose the document image, which is developed by a magenta (M) developer, and the resulting magenta visible image is transferred to transfer paper. Next, the photosensitive member is exposed through a blue (B) filter in the same way as above. Development is then made by use of a yellow (Y) developer and the resulting yellow visible image is transferred to the transfer paper in registration with the magenta image that has already been transferred on the transfer paper. Furthermore, the same process is repeated by use of a red (R) filter and a cyan developer, followed by the same procedure described above. The resulting cyan visible image is transferred in registration with the two images described above, and the resulting image is fixed, thereby providing finally a color print.
Thus, in accordance with this method, BGR color separation is applied to the document image, and the photosensitive member is exposed for each color. YMC (and black, if necessary) development is then effected, and the toner image for each color is thereafter transferred one by one in registration with the image of the other colors to the transfer paper on the transfer drum to obtain a full color image.
Color printers of the electrophotographic system for obtaining color hard copies are also known, as described in Japanese Patent Laid-Open No. 144452/1981, for example. In accordance with this prior art reference, an image of each color signal is written on a photosensitive member 301, that is uniformly charged with a charging device 302, with laser l.sub.1, l.sub.2, l.sub.3 as exposure means, as shown in FIG. 29 of the drawings. Symbols D.sub.1,D.sub.2,D.sub.3 represent developing devices for respective colors. When the latent images written with the lasers l.sub.1, l.sub.2, l.sub.3 are developed by toners having mutually different colors and are altogether transferred to transfer paper 303 by a transfer electrode 304, a multicolor hard copy can be obtained at a rate of one sheet per rotation of the photosensitive member.
In the prior art example described above, the image is written with three lasers, but the Applicant of the present invention proposed previously a construction wherein the image is written by use of one laser, as disclosed in Japanese Patent Laid-Open Nos. 75850/1985 and 76766/1985. FIG. 30 illustrates the apparatus of Japanese Patent Laid-Open No. 75850/1985 as a representative. One writing means 334 and developing devices 335 to 338 containing yellow, cyan, magenta and black toners, respectively, are disposed around a photosensitive member 331. First of all, the photosensitive member 331 is charged uniformly by a charging device 333, and exposure and write of the image are then effected by use of a laser beam modulated by a yellow image signal. Development is carried out in the developing device 335 containing the yellow toner. After the entire surface of the photosensitive member is charged in the next rotation of the photosensitive member 331, exposure is made by a magenta signal and magenta development is made in the developing device 336. Then, the magenta toner image is formed superposedly on the yellow toner image. After charging of the photosensitive member, exposure by the cyan signal and cyan development are made and the resulting cyan image is then formed superposedly on the two color images. Thereafter, the image is transferred to transfer paper P by a transfer electrode 340 to obtain a color hard copy.
The electrophotographic color recording apparatus described above exposes separately photoelectric signals corresponding to the images that are to be developed in yellow, magenta and cyan, to write the images.
There is also known a method which comprises using additionally a color reader, passing a document image through color filters to expose it on the photosensitive member, and developing the image by developers of colors that are complementary to the color filters used. Besides this method, there is known another method which comprises passing the document image through color filters, projecting the image on photo-conversion elements arranged unidimensionally such as line sensors to pick up electric outputs from the line sensors, modulates laser beams by the signals and records the image of each color in accordance with the electrophotographic system.
This processing system is fundamentally the same as the one used in processing image signals, such as color printing, color scanner, and the like, as color image processing technique in a broad sense of the word. Namely, in order to form an image, three color informations are obtained by color separation into three primary colors (B, G, R), and color reproduction is then made by arbitrary combinations of these color informations.
Besides the method described above for obtaining the color information by color separation into the three primary colors, there are known several other methods, though specific to some extents, by which color is separated into two primary colors to be read and read signals are obtained only for the limited colors.
Examples of such methods include one disclosed in Japanese Patent Laid-Open No. 162755/1981, the one shown in FIG. 10 and disclosed in Japanese Patent Laid-Open No. 44825/1982 and the technique reported by Nippon Telephone and Telegraph Co. in the above-described Preprint of the General Meeting of the Institute of Electronics and Communication Engineers of Japan.
Still another example of the prior art methods is one comprising separating a color into two primary colors with a half mirror and a filter to obtain a color signal on the basis of outputs of a unidimensional photosensor obtained by each of the two separated colors.
The electrophotographic reproducing machine described above is not different from an ordinary monochromic electrophotographic reproducing machine from the viewpoint of the reproduction of each color. Therefore, image density adjustment and fog prevention technique can be applied.
For example, Japanese Patent Publication Nos. 2550/1969, 4337/1974 and 92256/1979 use light as means for somehow compensating for changes in the characteristics of a photosensitive member, Japanese Patent Laid-Open Nos. 36129/1975 and 81237/1975 use corona discharge as the means described above and Japanese Patent Laid-Open Nos. 31942/1973 and 146629/1978 use both of the means. Furthermore, Japanese Patent Laid-Open Nos. 99824/1977, 110046/1977 and 32153/1981 compensate for changes in the characteristics in accordance with the copying time or the number or copies, and Japanese Patent Laid-Open Nos. 87174/1980, 83839/1979 and 98830/1978 control the image density by use of any factors by sensing the factors relating to the image formation process.
When a color reproducing machine based on the electrophotographic system described above is constituted several problems that have not occurred in the monochromic electrophotographic reproducing machine develop and it becomes difficult to obtain a color image always stably with high reproducibility.
For example, the photosensitive member and the developer undergo fatigue or change with time in the course of use of the reproducing machine, and reproducibility of the image tends to get worse. However, if the drop of reproducibility varies from color to color, color reproducibility of a superposed toner image will change when the toner image is superposed with different colors. Color reproducibility will change, too, due to the influences of changes in an exposure quantity and in the charging condition. Stability of the reproducing machines of the systems described above is not good particularly for the full color process. In order to solve these problems, advanced and complicated control is necessary, and the reproducing machine will become great in size and high in the production cost.
In the electrophotographic reproducing machine, a blackish white portion called "base fog" occurs when reproducing a white portion of the document, due to a small amount of toner that attaches to the white portion. This base fog occurs when a residual potential of the photosensitive member becomes great under the condition where the toner must not attach to the photosensitive member. The base fog is ordinarily prevented by applying a bias voltage to neutralize the residual voltage.
The base fog is not much offensive to the eye in the monochromic reproducing machine, but if it occurs in the color reproducing machine, it becomes very offensive because of color mixing and gives a feeling of disagreeableness just like the deposition of serious contamination.
From this aspect, prevention of the occurrence of the base fog is particularly important in the color reproducing machine. Since the occurrence of the base fog varies from color to color, it is extremely difficult to completely prevent it for all colors, and development of effective means has been much desired. Particularly in the case of business documents having colored portions at part of black characters or black-and-white picture, a copy with the base fog or a copy having a low contrast is not desirable, and such problems must be solved by all means.
Among color extraction systems for providing color signals to an image formation apparatus provided with a writing mechanism, as shown in FIGS. 29 and 30, the methods of FIGS. 9 and 10 having a reduced number of colors are more economical but the resulting color print is extremely ugly. Namely, when applying these methods in printing a document composed of a major portion of images having a black-and-white intermediate tone and a minor portion which is colored, the method of FIG. 9 provides a copy in which the intermediate tone portion is colored in violet and which gives an extremely different impression as to the color from that of the original and is quite unnatural. When the method of FIG. 10 is used, the portion of the intermediate tone is colored in green, and the resulting copy is extremely unnatural, too.
The full color process described above has low reproducibility of light colors by writing utensils called "marker pens" that are recently used for underlining documents or for coloring graphs, and the resulting copy has turbid colors of the portions colored by the marker pens or is devoid of the portions of the marker pens themselves.